The present invention relates to a method of controlling a DC electric motor, particularly an electric motor for the propulsion of an automotive vehicle, and to a device for carrying out this method.
One of the main drawbacks of electric traction automotive vehicles powered by storage batteries is their small cruising range. Attempts have been made to increase this cruising range by all means possible, e.g., by increasing the storage capacity per unit of weight of the batteries, by reducing the weight of the vehicles, and by improving the efficiency of the traction (driving) apparatus. In order to increase this efficiency, brake and deceleration energy recovery devices have been provided. The known energy recovery devices are complicated and frequently employ an auxiliary generator in order to be able to recover the maximum amount of energy, which, of course, increases the weight and the cost price of the automotive vehicle.
Furthermore, the known methods for controlling electric motors do not make it possible to obtain optimum operation at all modes, both in traction and in the recovery of energy.
Accordingly, an object of the present invention is a method of controlling an electric motor, in particular a DC electric motor for propelling an automotive vehicle, which permits a simple progressive and uninterrupted control and does not employ an auxiliary generator in order to be able to recover the maximum amount of braking and decelerating energy.
Another object of the present invention is a method of controlling an electric motor which makes it possible to adapt the characteristics of a DC motor to the different loads imposed at all speeds of rotation in the traction mode and to adapt the characteristics of said motor, used as a generator in the energy recovery mode, to the different resistance torques at practically all speeds of rotation.
Another object of the present invention is an apparatus for carrying out said method, which apparatus is as low in weight and cost as possible.
In accordance with the process of the present invention, the armature current of the electric motor is determined and the field current is regulated progressively in a continuous manner by cutting the current as a function of the speed of rotation of the motor by means of a control device which is common to both the traction and energy recovery modes, the motor being preferably of the separately excited type.
The apparatus for controlling the field current comprises a first signal chopper having a control input which produces substantially rectangular signals of preferably constant conduction time and of variable frequency of recurrence. In order to control the armature current, a second signal chopper having a control input and connected to a first amplifier in a traction mode and to a second amplifier in energy recovery mode is provided, the latter chopper producing substantially rectangular signals preferably of fixed frequency of recurrence and variable conduction time.
In accordance with a preferred embodiment of the invention, the armature current and field current choppers are controlled by a common control device, preferably via photocouplers whose emitting portions are arranged in a bridge circuit, the switching between the traction mode amplifier and the energy recovery mode amplifier being controlled by a device which detects the passage through zero of the armature current.
In accordance with the preferred embodiment of the invention, the substantially rectangular signals of fixed frequency of recurrence and variable conduction time which control the two armature current amplifiers are produced by a generator device comprising a comparator, one of the inputs of which is connected to a triangular voltage generator of fixed frequency of recurrence and the other input of which is connected preferably to the receiving device of a photocoupler whose emitting device is connected to the common control device.
In accordance with this same preferred embodiment of the invention, the emitting devices of the photocouplers controlling the field and armature current signal choppers are arranged in a bridge circuit, the input of this circuit receiving, on the one hand, a control current and, on the other hand, a signal proportional to the armature current, the diagonal of the bridge containing a photocoupler emitter which controls the variations in conduction time of the nearly rectangular signals of the armature current chopper, both in acceleration and in deceleration. A first branch of the bridge, which receives the signal proportional to the armature current, contains another photocoupler emitter which controls the variable frequency generator determining the field current, while a second branch of the bridge, which receives the control current, contains yet another emitting device of a photocoupler controlling the generator of variable recurrent frequency determining the field current. The two other branches of the bridge contain unidirectional conduction devices, e.g., diodes, connected in such a manner at to cause a unidirectional current to pass through the diagonal containing the emitter device of the photocoupler controlling the armature current chopper. Thus with a single photocoupler device, one can control the conduction time of the control signals of the armature current signal choppers both in acceleration and in deceleration and braking.